The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to an improved method for forming an isolation region.
Since the capacity of semiconductor integrated circuits has been increased and their functions have been varied, semiconductor integrated circuits tend to be made in larger scale. In accordance with the large integration of the circuits, it is desired that the dimensions of semiconductor elements be minimized. The size of the elements is required to be 3 .mu.m, 2 .mu.m and even on the order of submicrons.
A common technique for minimizing the element size is to isolate the elements by a dielectric material. A selective oxidation technique as one of the techniques for this purpose has been conventionally adopted.
A process for forming an isolation region by the selective oxidation technique in the process for manufacturing an npn bipolar type integrated circuit will be described with reference to FIGS. 1A to 1C.
(I) Referring to FIG. 1A, an n.sup.+ -type buried layer 2 is selectively formed on one major surface of a p-type silicon substrate 1. Thereafter, an n-type silicon epitaxial layer 3 is grown thereon. An oxide film 4 is then formed on the surface of the n-type silicon epitaxial layer 3 by thermal oxidation. Further, a silicon nitride film 5 is deposited on the oxide film 4. Opening 6 is selectively formed by photolithography for a prospective area for the isolation region of the oxide film 4 and the silicon nitride film 5.
(II) Referring to FIG. 1B, the exposed portion of the n-type silicon epitaxial layer 3 is selectively etched to form a groove 7 using the silicon nitride film 5 and the oxide film 4 as masks. In the same manner, boron is ion-implanted using the silicon nitride film 5 and the oxide film 4 as the masks to form a boron-implanted layer 8 in the region of the n-type silicon epitaxial layer 3 in the vicinity of the bottom of the groove 7.
(III) The wafer is oxidized in a wet atmosphere at high temperature using the silicon nitride film 5 as the oxidation-resistive mask. Therefore, the inner surface of the groove 7 is selectively oxidized to form an oxide film isolation layer 9, as shown in FIG. 1C. Simultaneously, boron in the boron-implanted layer 8 is diffused to form a p.sup.+ -type channel stopper 10. After the silicon nitride film 5 and the oxide film 4 are removed, a p-type base region is formed in the n-type silicon epitaxial layer 3 as an island shape isolated by the oxide film isolation layer 9 in accordance with the conventional process. Further, an n.sup.+ -type emitter region is formed in the p-type base region. An n.sup.+ -type collector electrode region is formed on the n-type silicon epitaxial layer 3, thus completing an npn bipolar integrated circuit.
In the selective oxidation method as described above, thermal oxidation must be performed at a high temperature for a long period of time. Therefore, oxidation progresses in the transverse direction through the oxide film 4 under the silicon nitride film 5. In other words, side oxidation occurs and a bird's beak 11 and a bird's head 12 are formed. The oxide film 4 is formed so as not to produce oxynitride from the silicon nitride film 5. The formation of the bird's beak 11 narrows the element formation region and results in a large dimensional change of the element formation region. Further, the pattern precision of openings is degraded and small openings are difficult to form. The formation of the bird's head 12 results in a level difference between the bird's head 12 and the n-type epitaxial layer 3, thus resulting in disconnection of an interconnection electrode layer formed above the bird's head 12 and the n-type epitaxial layer 3.
Further, the wall surface of the groove 7 is oxidized in the lateral direction in the same manner as in the direction of depth of the groove 7. The width of the oxide isolation layer 9 is made to be equal to the sum of the width of the opening of the groove 7 and twice the thickness of the isolation layer 9. In addition to the decrease of the integration density which is caused by the bird's beak as described above, the integration density is thus further lowered.
In the selective oxidation method, the element characteristics are adversely affected. When thermal oxidation is performed in an oxygen atmosphere of high temperature, using the silicon nitride film 5 as the mask, stress acts between the silicon nitride film 5 and the n-type silicon epitaxial layer 3. Further, thermal strain occurring on the n-type silicon epitaxial layer 3 produces crystal defects such as OSF (oxidation induced stacking faults) and the like in the n-type silicon epitaxial layer 3 around the oxide film isolation layer 9, thus degrading the element characteristics.
In order to solve these problems, a method is proposed in IBM Technical Disclosure Bulletin Vol. 22, No. 7, pp. 2749-2750 (December, 1979). This method will be described with reference to FIGS. 2A to 2D.
The n.sup.+ -type buried layer 2 is formed on the p-type silicon substrate 1. Thereafter, the n-type epitaxial layer 3 is grown on the n.sup.+ -type buried layer 2. The (underlying) oxide film 4 is formed on the surface of the n-type epitaxial layer 3 to a thickness of about 100 .ANG.. The (first) silicon nitride film 5 of a thickness of about 1,000 .ANG. is deposited on the underlying oxide film 4. A desired opening is formed in the first silicon nitride film 5 and the underlying oxide film 4. Thereafter, the exposed part of the n-type epitaxial layer is etched to form the groove 7, using the first silicon nitride film 5 as the mask. At this time, the first silicon nitride film 5 and the oxide film 4 extend in the form of an over-hang (FIG. 2A). Thermal oxidation is performed to form an oxide film 13 of a thickness of about 100 .ANG. on the inner surface of the groove 7. A second silicon nitride film 14 is formed to cover the entire surface including the groove 7. Reactive ion etching is performed, using the over-hang of the first silicon nitride film 5 as the mask, in order to remove the second silicon nitride film 14 at the bottom of the groove 7 in a self-aligned manner (FIG. 2B). In this manner, the self-aligned silicon nitride film is formed on the side wall of the groove 7. Thereafter, a p.sup.+ -type impurity is ion-implanted in the bottom of the groove 7 as needed. Therefore, within the groove 7, an oxide isolation layer 9' and a p.sup.30 -type channel stopper 10 are formed (FIG. 2C). The first silicon nitride film 5, the second silicon nitride film 14 and the underlying oxide film 4 are then etched (FIG. 2D).
In the above method, since the self-aligned silicon nitride film which is the oxidation-resistive film is formed on the side wall of the groove 7 by reactive ion etching, side oxidation in the groove 7 is minimized. In particular, an oxidizing agent forms a thick oxide film in the bottom of the groove 7. Further, the oxidizing agent permeates from the bottom of the groove 7 along the oxide film 13 of the side wall of the groove 7. The thus formed oxide film pushes up the second silicon nitride film 14. In this manner, the oxide isolation layer 9' is formed to cover the groove 7. The level of the oxide isolation layer 9' is the same as that of the surface of the n-type silicon epitaxial layer 3. The produced bird's beak is small. However, even if the above method is adopted in order to minimize further the elements, the groove 7 must be filled with the oxide in order to equalize the level of the oxide isolation layer and the n-type silicon epitaxial layer. Therefore, a bird's beak is more or less produced on the oxide isolation layer. It is known that the bird's beak may be minimized by making the first silicon nitride film as the oxidation-resistive mask material thick or by making the underlying oxide film under the first silicon nitride film thin. However, when the first silicon nitride film is made thick and the underlying oxide film is made thin, a crystal defect caused by thermal strain tends to occur in the n-type silicon epitaxial layer. Therefore, this method cannot completely eliminate the bird's beak.
In order to minimize the enlargement of the oxide isolation layer 9' which is caused by the bird's beak and to prevent the formation of the crystal defect in the n-type silicon epitaxial layer 3 around the oxide isolation layer 9', a very thin second silicon nitride film 14 must be formed.
In the method as shown in FIGS. 2A to 2D, since reactive ion etching is performed, using the overhang of the first silicon nitride film 5 as the mask, a very thin silicon nitride film 14 can be properly formed on the side wall of the groove 7. However, the formation of the overhang of the first silicon nitride film 5 is performed by undercutting when the groove 7 is formed by etching. Therefore, the groove 7 is enlarged in the transverse direction to the extent corresponding to the length of the overhang. The enlargement of the groove 7 results directly in the enlargement of the oxide isolation layer 9', degrading the compactness of the elements.